Sweating in a human is to a large extent controlled from the hypothalamus through autonomic pathways and the sympathetic nervous system to the skin everywhere in the body. It is thus a very central regulation, and large synchrony between different skin sites is to be expected. The main purposes of sweating are for the a) thermal balance of the body, b) obtaining best frictional properties of hands and feet, c) psychosomatic functions related to e.g. nervousness (“cold” sweating), gustatory sweating during eating or sweating correlated to face flushing.
Some persons have problems with too little or too strong sweating, locally or affecting larger parts of the body. The situation of too much sweating may be so serious that it is classified as a disease: hyperhidrosis. Four classes of therapy are used: treatment with aluminium salts, dc current through the affected skin area, sympatectomi and injection of botulintoxin. As the last two methods are invasive, and as many patients are not satisfied with their therapy, there is an ongoing search for reliable diagnostic methods and tools. Up to now simple and robust ambulatory measurements of sweat activity on several skin sites simultaneously have not been possible.
In the prior art several solutions for measuring skin moisture (i.e. hydration, water content, dryness) by utilizing skin impedance measurements are known.
U.S. Pat. No. 5,738,107 describes the measurement of skin moisture by an electrical susceptance method and the use of at least two sensors (electrodes).
European patent application EP 0 315 854 A1 describes the measurement of skin moisture in the keratinous layer (=stratum corneum) by measuring impedance. The measurements are switched so that both the surface and the depth of the keratinous layer can be measured.
International Patent Publication WO 92/06634 describes depth selective, localized measurements of electric impedance in organic and biological materials. The depth selectivity is obtained by inserting an actively driven third electrode between two measuring electrodes.
Japanese Patent Abstract JP 2005052227 A relates to an instrument measuring the water content in the stratum corneum,
Another Japanese Patent Abstract JP 07303618 A relates to an apparatus for the measurement of skin impedance.
In the article “An instrument for the evaluation of skin hydration by electrical admittance measurement”, by Ø. G. Martinsen & al, in Innov. Tech Biol. Med., Vol. 14, No. 5, 1993, p. 589-596 and in the article “Continuous estimation of parameters in skin electrical admittance from simultaneous measurements at two frequencies”, by L. Mørkrid and Z.-G. Qiao, in Medical & Biological Engineering & Computing, November 1988, pp. 633-640, there are described electrical impedance measurement at frequencies ranging from 60 Hz to 1 kHz, and in the range 1-1000 kHz, respectively.
The publication “Continuous monitoring of sweating by electrical conductivity measurement” by A. K. M. Shamsuddin and T. Togawa, in Physiol. Meas., Vol 19, 1998, pp 375-382, describes a method of measuring changes in the conductivity of perfusing water using a chamber attached to the skin surface. This device requires a source of an ion-free solution and an arrangement of channels for guiding the solution past the skin surface, a solution which is not very attractive for ambulatory use.